An analog input unit is a device having an A/D conversion function to convert analog values to digital values. The conversion characteristic of an analog input unit for converting analog values to digital values is usually indicated by a straight line connecting two points on a two-dimensional graph having an analog-value axis and a digital-value axis. For example, after the conversion characteristic is calibrated when the analog input unit is shipped as a product, even if the user desires to change the conversion characteristic, only the gradient of the straight line can be adjusted in most cases. Further, to change the conversion characteristic significantly, computing of analog values needs to be incorporated into a program running in the CPU unit that controls a PLC. In such a case, the load for creating the program increases.
A/D conversion in an analog input unit involves computing being performed by a program running in the CPU unit. Thus, the processing time for A/D conversion depends on the control cycle (scan time) of the CPU unit. Consequently, it is difficult for the analog input unit to sufficiently exhibit high-speed and fixed-cycle input/output performance. A similar problem occurs with an analog output unit. An analog output unit is a device having a D/A conversion function to convert digital values to analog values.
An analog input/output unit is a device having both an A/D conversion function and a D/A conversion function. In some analog input/output units, the conversion characteristic for converting analog input to analog output is indicated by a continuous straight line connecting a plurality of variable points. For example, the number of variable points is generally up to approximately ten. Also in this case, to realize any further significant change in the conversion characteristic, computing of analog values needs to be incorporated into a program running in the CPU unit that controls a PLC.
The coordinates of a variable point are usually set as numerical values. When the conversion characteristic is adjusted by setting numerical values, it is difficult to visually image the conversion characteristic. When computing of analog values is incorporated into a program running in the CPU unit, it is also difficult to check the conversion characteristic visually. Therefore, operations for adjusting the conversion characteristic and debugging operations become complicated.